Two Paths to Low Mass

A brown dwarf (also known as an infrared dwarf) is, in a way, a failed star. Early in their lives, these ultra-low-mass stars (13+ MJ) fuse deuterium into helium-3, and in the highest mass brown dwarfs (65-80 MJ) lithium is depleted into helium-4, as shown below.

But the mass is too low for fusion to be sustained (the temperature and pressure in the core aren’t high enough), and soon the fusion reactions peter out. Then, only the slow process of thermal contraction provides a source of heat for the wanna-be star.

There is another, very different, path to a brown dwarf star. A cataclysmic variable usually consists of a white dwarf and a normal star in a close binary system. As material is pulled off the “donor star” (as the normal star is called) onto the white dwarf, the donor star can eventually lose so much mass that it can no longer sustain fusion in its core, and it becomes a brown dwarf star.

When we see a white dwarf / brown dwarf binary system, how do we know that the brown dwarf wasn’t always a brown dwarf? Strong X-ray and ultraviolet emission provides evidence of an accretion disk around the white dwarf, and astronomers can calculate the rate of mass transfer between the two stars. Often, this is billions of tons per second! Using other techniques to estimate the age of the binary system, we sometimes find that the donor star must have started out as a normal star with much more mass than we see today.